Method of reinforcing a plastic pallet

ABSTRACT

A pallet assembly includes a top deck pallet portion and a bottom deck pallet portion which are mounted to each other and have a plurality of column portions extending therebetween. At least one of the top deck pallet portion and bottom deck pallet portion has a first surface with a plurality of cross-ribbed members projecting therefrom. The pallet assembly further includes at least one stiffening member securely mounted to the plurality of cross-ribbed members to define a plurality of box beam sections between the at least one stiffening member and the at least one of the top deck pallet portion and bottom deck pallet portion.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.09/108,532 filed Jul. 1, 1998, now U.S. Pat. No. 6,250,234.

TECHNICAL FIELD

The present invention relates to a method of reinforcing a plasticpallet, and more particularly to a method of reinforcing a plasticpallet by applying a plurality of sheet strips along relatively weakstructural portions of the pallet to form a plurality of substantiallyrectangular hollow vertical cross-sections along the length of therelatively weak structural portions for improved stiffness.

BACKGROUND ART

Replacing wood pallets with plastic pallets has been a goal for manyyears. The advantages of the plastic pallets are many as compared towood, including greater durability, lighter weight, more consistentdimensions, improved cleanliness, water resistance, higher residualvalue for recycling, and no nails which may damage products beingsupported thereon.

One major hurdle to overcome with plastic is the cost. Plastic palletsare more expensive than wood by three to five times. This cost can beoffset by the number of trips or shipments that can be achieved withplastic versus wood pallets. Another major hurdle is the stiffness ofplastic pallets. Racking loaded pallets in warehouses for up to 30 daysis common, and the combination of low tensile strength and creep limitthe use of plastic.

There are three conventional methods of overcoming these weaknesses. Thefirst is to add reinforcement such as steel or a composite to thepallet. This generally adds significant cost and weight and complicatesrecycling of the pallet. The second is to make the pallet taller. Thislimits the height of product to be stacked on the pallet. The third isto use reinforced or engineered resins. Again, this adds significantcost and weight. All three obviously limit the acceptance of plasticpallets.

U.S. Pat. No. 3,580,190 provides a partial solution to the stiffnessproblem by attaching top and bottom sheets 22,24 to the structuralnetwork 23, as shown in FIG. 1 thereof. However, this solution does notresolve the bending stiffness problem because large lateral andlongitudinal unsupported areas still exist, such as in areas 26, 37, 38,49 and 50. In other words, this design merely further stiffens thesupport column areas 67, 68, 69, 97, 98, 99, 28, 30, 32, which alreadyprovide substantial stiffness merely as a result of their height. Theweakness of this design is apparent in column 6, lines 60-71, whereFowler recommends the use of a material having a flexural modulus (orYoung's modulus) greater than about 200,000 psi. Such a high modulusmaterial is apparently required because the structure described does notprovide significant resistance to deflection along the length and widthof the pallet. High modulus materials add substantial cost to thepallet.

Further complicating the problem, modern pallets typically require largeopenings for receipt of pallet jacks. For example, the pallet shown inFIGS. 1-3 includes a top deck portion 16 supported on a plurality ofsupport columns 18, which are attached to support rails 20, which formthe bottom deck 19. Such structure cooperates to form two large openings11,13 on each side of the pallet 10, as well as four bottom openings 15formed in the lower deck 19. In this configuration, the rails 20 of thelower deck 19 are typically structurally weak, resulting in poordeflection stiffness. Such problems have proven very difficult toovercome because of the very thin nature of the lower deck 19.Similarly, the thin design of the top deck 16 results in the samedeflection problem between columns 18.

Because pal lets are exposed to significant abuse, any solution to thestiffness problem must not adversely effect the impact strength of thepallet.

Accordingly, a need exists for improving the stiffness of modern plasticpallets configured to receive a pallet jack, without reducing impactstrength of the pallet.

SUMMARY OF THE INVENTION

The present invention provides a method of reinforcing a modern plasticpallet by affixing sheet strips along relatively weak structuralportions of the pallet to form a plurality of substantially rectangularhollow vertical cross-sections along the length of the relatively weakstructural portions for improved stiffness without loss of impactstrength.

More specifically, the present invention provides a method ofreinforcing a plastic pallet having a thin top deck portion, a pluralityof support columns extending from the top deck portion and a pluralityof support rails connected to the support columns to form a thin bottomdeck portion, wherein the support rails each include a sheet portionwith a plurality of vertical ribs extending therefrom. The methodincludes the step of welding a plurality of plastic sheets to thevertical ribs between the support columns to form a plurality ofsubstantially rectangular hollow vertical cross-sections along thelength of the support rails for improved stiffness. It is contemplatedthat the substantially rectangular hollow vertical cross-sectional areasmay be filled with a secondary material, such as structural foam forimproved structural integrity.

Accordingly, an object of the present invention is to provide a methodof structurally reinforcing a modern plastic pallet configured toreceive a pallet jack, in a manner which improves stiffness without lossof impact strength.

The above object and other objects, features and advantages of thepresent invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a modern plastic pallet in accordancewith the present invention;

FIG. 2 shows a bottom exploded perspective view of the pallet of FIG. 1;

FIG. 3 shows a top exploded perspective view of the pallet of FIG. 1;

FIG. 4 shows a cut-away perspective sectional view of a pallet inaccordance with an alternative embodiment of the invention;

FIG. 5 shows a top exploded perspective view of a pallet in accordancewith a second alternative embodiment of the invention; and

FIG. 6 shows a bottom exploded perspective view of the pallet of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In a racking scenario, the modern plastic pallet 10 (which is configuredto receive a pallet jack from any side), shown in FIG. 1, is supportedfrom below on two opposing edges 12,14, and loaded on the top deckportion 16. The pallet must support this load with a minimum ofdeflection. The top deck portion 16 is supported by a plurality ofsupport columns 18 extending from the top deck portion 16 and attachedto the support rails 20, which form the bottom deck 19. The supportrails 20 are generally relatively weak structural portions of the palletbecause they are thin in vertical cross-section and are supported onlyat opposing ends by the columns 18. Accordingly, the support rails 20tend to deflect when the pallet is loaded.

Using simple beam formulas it is known that the deflection increases asthe load or the span distance increases and decreases as the materialmodulus (E) or section moment of inertia (I) increases. Since the loadand the span are defined for a given application, the variables used tominimize deflection are the material and the section design.

A good rule of thumb with plastics is that as the E value increases fora given material, the impact strength decreases. There are engineeredmaterials that can solve these problems but they are too expensive forwide spread use. The most common method is to use a commodity resin suchas polyethylene or polypropylene and add a filler to stiffen the resin.Fillers add weight and reduce impact strength for a given material aswell as impact the recyclability in some cases. They also add cost,which can be the biggest problem to overcome. So, the ideal pallet woulduse a commodity resin because of cost, weight, and impact strength.Accordingly, optimizing the design of the pallet is the preferred methodto achieve improved pallet performance.

As mentioned above, deflection decreases as the section moment ofinertia (I) increases. For a pallet, the easiest method to increasestiffness is to increase height. However, in practice there is a maximumallowable height for pallets, and existing pallets are generallydesigned at this maximum value. Accordingly, the only alternative is tomaximize the moment of inertia for each pallet component, namely the topdeck 16 and bottom deck support rails 20.

For a given section geometry, the highest I value is for a solidsection. For instance, the stiffest top deck design is one that is asolid plastic. Obviously, this is impractical because of weight andcost. Most designs attempt to overcome this by using ribbed sectionsinstead of solid sections to minimize the loss of I and reduce theweight to an acceptable level. Another method is to use foaming agentsalong with the ribs to minimize the weight of the ribs and improve the Ivalue. Both methods have a limit to their effectiveness. Namely, ribsare not the ideal geometry to maximize the I value, and while foamingimproves this slightly, it also reduces the impact strength of thematerial.

It is known that for a given section the material closest to the neutralaxis has the least effect on the I value, and the material farthest awayhas the greatest effect. In other words, a hollow or I-beam section isstiffer than a rectangular section of equal height and area. Therefore,the object of the design is to create hollow or boxed sectionseverywhere possible. Conventional injection molding techniques make thisalmost impossible to create, but by using a simple secondary operation,we are able to make a boxed top deck 16 and bottom deck 19.

A pallet in accordance with the present invention includes the top deckportion 16, which is injection molded conventionally and consists of aflat upper surface 22 with a series of ribs 24 protruding from the uppersurface 22 as shown in FIG. 2. The top deck portion 16 includes aplurality of pockets 26 for receiving the support columns 18. Aplurality of plastic sheet strips 28 are sonically welded to the ribs 24to form a plurality of substantially rectangular hollow boxed sectionsbetween the pockets 26 within the top deck 16 (as described later withreference to FIG. 4). Alternatively, other attachment methods such asvibratory welding, hot plate welding, adhesive etc. may be used forattachment of the plastic sheet strips 28.

The bottom deck 19 is constructed similarly but has the support columns18 integrally molded therewith. Ribs 30 protrude downwardly from thesheet portions 32 of the support rails 20, and a plurality of plasticsheet strips 34 are welded to the ends of the ribs 30 to form aplurality of boxed cross-sections along the length of the rails 20between the columns 18. The top and bottom decks 16,19 may be joinedpermanently by welding, or can be snapped together as commonly known inthe art.

The method described above is preferably used to stiffen conventionalribbed pallet designs. The small sheets of plastic 34 are welded intocritical deflection areas of the existing pallets for stiffening. Also,new pallets could be designed to accept the sheets for applications thatrequire racking, and would eliminate the sheets for lighter, lower costapplications. For example, the ribs 30 may be recessed in order toreceive the sheets 34 in a position flush with the bottom surface of thesupport rails 20.

The method described above is particularly applicable for use in palletssuch as that shown in FIG. 1 which has a very thin top deck 16 andbottom deck 19 to allow four-way entry of pallet jacks. The methoddescribed may be used to maximize the moment of inertia of each deckmember.

Referring to FIG. 4, an alternative embodiment of the invention isshown. Similar to the embodiment shown in FIG. 1, the bottom deck rails40 include a sheet portion 42 with a plurality of vertical ribs 44extending therefrom. The plastic sheet strips 46 are welded to the ribs44 to form the plurality of substantially rectangular hollow verticalcross-sections 48 along the length of the support rails 40. Of course,numerous ribs 44 could be added to create numerous rectangularcross-sections for further improved structural integrity.

The pallet shown in FIG. 4 differs from the earlier embodiment describedwith reference to the FIGS. 1-3 in that a large sheet 50 is welded tothe ribs 52 across the breadth of the upper deck 54 for improvedstructural integrity of the upper deck 54.

It is contemplated that good results could be achieved even by onlywelding the peripheral ribs to the plastic sheet strips. It is furthercontemplated that the plastic sheet strips need not be welded, but couldbe affixed in any manner, such as adhesive, etc. It is also contemplatedthat the sheet strips need not be plastic.

Referring to FIGS. 5 and 6, a second alternative embodiment of theinvention is shown. In this embodiment, the pallet 110 includes a thintop deck 116 connected to a thin bottom deck 119 by nine support columns118. The bottom deck 119 is comprised of a plurality of support rails120 which extend between the columns 118. Each support rail 120 includesa sheet portion 122. Because each support rail 120 forms a relativelyweak structural portion of the pallet 110, an extruded plasticrectangular tube 127 is welded against each respective sheet portion 122to add stiffness to each support rail by forming substantiallyrectangular vertical cross-sections along the length of each supportrail 120. Similarly, the top deck 116 includes open channels 128adjacent the top sheet 130, and an extruded plastic rectangular tube 131is welded within each channel 128 against the top sheet 130 between thecolumns 118 to form substantially rectangular vertical cross-sectionsalong the length of each channel 128 between the columns 118 forimproved stiffness. In this configuration, the rectangular tubes 127,131may be inexpensively extruded, and add substantial structural integrityto the pallet 110 without limiting the pallet's ability to receivepallet jacks from any side thereof.

While the best mode for carrying out the invention has been described indetail, those familiar with the art to which this invention relates willrecognize various alternative designs and embodiments for practicing theinvention within the scope of the appended claims.

What is claimed is:
 1. A pallet assembly comprising: a top deck palletportion and a bottom deck pallet portion mounted to each other andhaving a plurality of column portions extending therebetween, wherein atleast one of the top deck pallet portion and bottom deck pallet portionhas a first surface with a plurality of cross-ribbed members projectingdownwardly therefrom; and at least one stiffening member securelymounted to the plurality of cross-ribbed members to define a pluralityof box beam sections between the at least one stiffening member and theat least one of the top deck pallet portion and bottom deck palletportion.
 2. The pallet assembly of claim 1, wherein the at least onestiffening member comprises a plastic sheet.
 3. The pallet assembly ofclaim 1, wherein the at least one stiffening member comprises a tubemember.
 4. The pallet assembly of claim 1, wherein the first surface isdefined by a lower surface of the at least one of the top deck palletportion and bottom deck pallet portion.
 5. The pallet assembly of claim1 wherein the at least one stiffening member is securely mounted to theat least one of the top deck pallet portion and bottom deck palletportion by introducing heat therebetween, plasticizing each surface,compressing the surfaces together, and allowing them to cool.
 6. Apallet assembly comprising: a first pallet portion having a firstsurface defined by a first plurality of cross-rib members; a firstplurality of stiffening members mounted to the first plurality ofcross-rib members to form first box beam sections therebetween; a secondpallet portion mounted to the first pallet portion by a plurality columnportions extending between the first pallet portion and second palletportion, the second pallet portion having a second surface defined by asecond plurality of cross-rib members; and a second plurality ofstiffening members mounted to the second plurality of cross-rib membersto form second box beam sections therebetween.
 7. The pallet assembly ofclaim 6, wherein the first plurality of stiffening members compriseplastic sheets.
 8. The pallet assembly of claim 6, wherein the firstplurality of stiffening members comprise tube members.
 9. The palletassembly of claim 6, wherein the second plurality of stiffening memberscomprise plastic sheets.
 10. The pallet assembly of claim 6, wherein thesecond plurality of stiffening members comprise tube members.
 11. Apallet assembly comprising: a first pallet portion formed of plastic andincluding a top member and a plurality of top intermediate stiffeningmembers, each having a corresponding first mating surface which aresecured together to integrally define a first plurality of box-beamsections therebetween, wherein at least one of the corresponding firstmating surfaces is defined by a plurality of first cross-rib members;and a second pallet portion formed of plastic and including a bottommember and a plurality of bottom intermediate stiffening members, eachhaving a corresponding second mating surface which are secured togetherto integrally define a second plurality of box-beam sectionstherebetween, wherein at least one of the corresponding second matingsurfaces is defined by a plurality of second cross-rib members; and aplurality of column portions extending between the first pallet portionand second pallet portion.
 12. The pallet assembly of claim 11, whereinthe top intermediate stiffening members comprise a plastic sheet. 13.The pallet assembly of claim 11, wherein the top intermediate stiffeningmembers comprise a tube member.
 14. The pallet assembly of claim 11,wherein the bottom intermediate stiffening members comprise a plasticsheet.
 15. The pallet assembly of claim 11, wherein the bottomintermediate stiffening members comprise a tube member.
 16. A palletassembly for storing and transporting objects thereon, comprising: afirst pallet deck comprising: a first pallet member having a first uppersurface upon which objects are placed and a first lower surface, whereinone of the first upper surface and first lower surface is defined by aplurality of first cross-rib members; at least one stiffening firstmember having a generally planar surface corresponding to the pluralityof first cross-rib members and securely mounted therewith to define afirst plurality of box beam sections therebetween; a second pallet deckattached to the first pallet deck; and a plurality of column portionsextending between the first pallet deck and second pallet deck.
 17. Thepallet assembly of claim 16, wherein the at least one first stiffeningmember comprises a plastic sheet.
 18. The pallet assembly of claim 16,wherein the at least one first stiffening member comprises a tubemember.
 19. The pallet assembly of claim 16 wherein the at least onefirst stiffening member is securely mounted to the first pallet memberby introducing heat therebetween, plasticizing each surface, compressingthe surfaces together, and allowing them to cool.
 20. The palletassembly of claim 16 wherein the at least one second stiffening memberis securely mounted to the second pallet member by introducing heattherebetween, plasticizing each surface, compressing the surfacestogether, and allowing them to cool.
 21. The pallet assembly of claim16, wherein the second pallet deck comprises: a second pallet memberhaving a second upper surface and a second lower surface, wherein one ofthe second upper surface and second lower surface is defined by aplurality of second cross-rib members; and at least one stiffeningsecond member having a generally planar surface corresponding to theplurality of second cross-rib members and securely mounted therewith todefine a second plurality of box beam sections.